One controversial modification you can do on a KLR650 is installing Watt-Man’s Thermo-Bob. In this post I will walk around the topic and find out what does it do and what are the pros and cons in installing it.

On a stock KLR when the engine starts to run, the cooling system kicks of immediately as well. Some claims it leads to problems because the engine warm-up time is much longer during winter time and later, the engine and coolant temperature is fluctuating in a wide range.

I can confirm only the later statement. When riding on the open road, the temperature gauge stays in the lower quarter of the temperature range but on a slower trail ride it moves to the upper half of the scale and the cooling fan kicks off as well. The same is true about open road ride vs. commuting.

What does the Thermo-Bob do? The Thermo-Bob is an external thermostat with a bypass in the cooling system. When the engine is cool, the Thermo-Bob opens the bypass and the warm coolant returns to the engine without passing through the radiator. It helps the engine to warm up quicker and the additional thermostat keeps the engine temperature more even. It also raises the operating temperature of the engine.


What the manufacturer is claiming as advantage?

  • More stable engine temperature will reduce the stress on the engine.
  • A warmer operating temperature and hotter oil temperature will extend the life of the engine.
  • The stable engine temperature and warmer oil will result in better fuel economy with 1-2 mpg.

Technically the Thermo-Bob does exactly what the manufacturer claims. The engine warms-up quicker in cold weather, and it keeps the operating temperature more stable on a higher temperature. It is all fine if you are living in Iceland, Sweden or Norway but what about countries with a warm climate, with hot summers or no real winter? What about if your riding is more slow riding, trail or gravel oriented?

I assume, the engineers at Kawasaki knew what they are doing and the engine, together with the cooling system was designed properly. This assumption is being supported by the fact, that Kawasaki manufactured roughly 150000 KLR650 by now, and we don’t hear about engine overheating or oil burning problems. So far what I see, the KLR650 is famous about the simple design, reliability and long life with high mileage without the Thermo-Bob.




Why not to install a Thermo-Bob then?

  • The engineers at Kawasaki put in enough tolerance into the design to cope with the temperature swings.
  • People with Thermo-Bob does not get better performance, mileage or reliability on the KLR.
  • It complicates a simple design by adding more points to fail to it. Extra hoses and hose joints to break or leak, the Thermo-Bob itself can fail and the cooling fan will run more often in warmer temperature.
  • New Zealand has a temperate climate without a real winter here on the North Island which makes the primary function of the Thermo-Bob unnecessary.

What is my verdict in the case of the Thermo-Bob? I think here on the North Island in New Zealand, the Thermo-Bob is unnecessary, but you will do no obvious harm to your bike if you install it.

Do you have a different opinion? We would like to hear it!


Mick sent his view on this topic to us. Here it is, unchanged, as it arrived into our “Inbox”. Thanks Mick.

Firstly great review & nice to see a unbiased opinion, as most people I know who have done the mod like telling people it’s a must do …. I don’t agree -particularly in milder climates . I also note that the bypass on the “bob” is always open even when the thermostat is open- so there is a unknown percentage of coolant not going through the radiator so I guess that is technically reducing your cooling capacity when you need it most…whether this translates to a issue in the field remains to be seen , I doubt “bob” wants to mention it ! I run a manual fan switch that I flick on when in soft sand, hard trails, etc and I never have a issue with high temps.
Regards Mick